The invention relates to a circuit arrangement for generating two signals having a phase difference of approximately 90 degrees. The invention is preferably applied in the modulator of a radio transmitter and/or in the demodulator of a radio receiver to generate the in-phase signal (I) and the quadrature signal (Q).
In radio receivers it is common to use a so called I-Q demodulator, where either the intermediate frequency signal or the local oscillator signal is divided into two signals having a phase difference of 90 degrees. The modulation methods are generally described i.a. in the following publications: 1! Seppo J. Halme, Televiestintajarjestelmat (Telecommunication Systems), 1992, Otatieto Oy, Espoo; and 2! Edward A. Lee, David G. Messershmitt, Digital Communication, 1990, Kluwer Academic Publishers, Boston.
In order to generate signals with different phases it is previously known to use i.a. passive RC circuits (FIG. 1) and divider circuits (FIG. 2) comprising discrete resistances and capacitances. The solution illustrated in FIG. 1 has RC circuits in two signal branches, whereby the first RC circuit R1-C1 creates a negative phase difference and the second RC circuit R2-C2 creates a positive phase difference regarding the input signal, so that with a suitable component design provides the desired phase difference between the output signals of the two signal branches. The above-mentioned RC circuits are used for substantially sinusoidal signals, because the desired phase difference is obtained only on a very narrow frequency range.
The solution embodied with RC circuits has a disadvantage in that it requires very accurate component values and small variations between individual components, and these characteristics are very difficult to obtain, particularly regarding resistors and capacitors in integrated circuits. If the required very low tolerances are not met, then the phase difference is not 90 degrees with a sufficient precision and the output signal amplitudes have a too large difference. An alternative is to use adjustable components, but an adjustment operation like this for each device causes substantial extra costs in the production of the devices. The solution realized with an RC circuit also has a disadvantage in that the phase difference and the output amplitude depend on the operating frequency and on the load.
In order to reduce the problems with RC circuits it is known to use a circuit arrangement, in which the capacitance C of the RC circuit is formed, at least partly, by a capacitance diode, and in which the phase difference is adjusted by controlling the voltage of the capacitance diode and thus its capacitance, e.g. with the aid of a processor and a D/A converter. The advantage of this solution is that the phase can be controlled automatically, but a disadvantage is the extra cost introduced by the controllable components. Further it is not easy to realize the entire circuit as an integrated circuit on semiconductor, such as silicon.
The solution shown in FIG. 2 for generating signals with different phases comprises a threshold means S and two bistable dividers F1 and F2. The output of the first divider circuit F1 changes its state at the rising edge of the input signal and the output of the second divider circuit F2 changes its state at the falling edge of the input signal, which creates a phase difference of 90 degrees between the output signals. The solution realized with divider circuits can be used only in connection with digital signals. The solution has also a disadvantage in that the signal frequency is halved, which requires the use of an input signal with double frequency. When high frequency signals are used it may be inconvenient to generate the double frequency i.a. due to the restricted operating frequencies of the components. If the circuit shall create an accurate phase difference it further requires that the pulse ratio of the input signal is exactly 50%.
Further it is known to generate a phase difference of 90 degrees using frequency multiplier, inverter and divider circuits. Then the frequency of the input signal is multiplied by two, the resulting signal is divided by two in the first signal branch (I), and correspondingly, in the second signal branch (Q) it is inverted and the frequency of the inverted signal is divided by two. The signals thus obtained have a phase difference of 90 degrees, if the performed operations were ideal. However, in practice there occurs phase delays in the signal path, so that it is very difficult to obtain a sufficiently accurate phase difference. Further the frequency multiplier and divider circuits consume a substantial amount of supply energy, which will shorten the operating time in battery and accumulator powered equipment.
The I/Q-demodulator solutions currently used for instance in digital radiotelephones require the use of a manually adjustable trimmer resistor or trimmer capacitor to control the phasing of the I/Q injection signals controlling the demodulator, because the components in the circuit have values varying within certain tolerances from one individual to the other. The trimmers cause extra costs in the production, further their adjustment is inconvenient, and mechanically adjustable components are also more unreliable when compared to fixed components.